Organometallic Compounds II(Part II)

B.Sc. (H) Chemistry

Dr. Rajanish N. TiwariDepartment of ChemistryMahatma Gandhi Central University

UNIT III

Infrared Spectra for organometallic compounds

The carbonyl group in organometallic compounds are

used to study by Infrared spectroscopy.

The position of the Infrared band can indicate the function of a

ligand (e.g., terminal vs. bridging modes).

In the case of π –acceptor ligands, can describe the electronic

environment of the metal.The peak appear in the range between 2100-1700 cm-1

Number of Infrared Bands

A molecular vibration is infrared active (i.e., excitation of

the vibrational mode can be measured via an IR spectrum

absorption) only if it results in a change in the dipole

moment of the molecule

It is often useful to consider a particular vibrational mode for a

compound.

For example, useful information often can be obtained from the

C-O stretching bands in infrared spectra of metal complexes

containing CO (carbonyl) ligands.

Selected Vibrational Modes

These complexes have a single C-O stretching mode

and consequently show a single band in the IR.

Monocarbonyl Complexes

Dicarbonyl Complexes

Two geometries, linear and bent, must be considered:

O – C – M – C – O

Two CO ligands arranged linearly, Only an antisymmetric vibration of theligands is IR active

Two CO ligands are oriented in a nonlinear fashion, both symmetric and

antisymmetric vibrations result in changes in dipole moment, and both are IR

active:

Two CO ligands are oriented in a nonlinear fashion, both symmetric and

antisymmetric vibrations result in changes in dipole moment, and both

are IR active:

Symmetric Stretch

Antisymmetric Stretch

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IR spectrum is a convenient tool for determining the structure of

molecules with two CO ligands: a single band indicates linear

orientation of the CO ligands, and two bands indicate nonlinear

orientation.

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For molecules containing two CO ligands on the same metal atom, the

relative intensities of the IR bands can be used to determine the

approximate angle between the CO ligands, using the equation

Where φ is the angle between ligands

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Complexes containing three or more carbonyls

The predictions for the Complexes containing three or more carbonyls

are not quite so simple.

The exact number of carbonyl bands can be determined according to the

symmetry approach. (not discussed here).

In carbonyl complexes, the number of C-O stretching bands cannot

exceed the number of CO ligands.

The alternative is possible in some cases (more CO groups than IR

bands), when vibrational modes are not IR active (do not cause a change in

dipole moment).

For convenient reference, the numbers of bands expected for a variety of COcomplexes are given in next slide.

Carbonyl stretching bands

Carbonyl stretching bands

Positions of IR Bands

The position of the carbonyl stretching band always provides usefulinformation.

In the case of the isoelectronic species [Mn(CO)6]+, Cr(CO)6, and [V(CO)6]-, anincrease in negative charge on the complex causes a significant reduction inthe energy of the C-O band as a consequence of additional π back-bondingfrom the metal to the ligands.

The bonding mode is also reflected in the infrared spectrum, with energy decreasing in the order

terminal CO > doubly bridging CO > triply bridging CO

The position of the carbonyl bands provide important clues to the electronic

environment of the metal.

The greater the electron density on the metal (and the greater the negative

charge), the greater the back-bonding to CO, and the lower the energy of the

carbonyl stretching vibrations.

The positions of infrared bands are also a function of other ligands present.

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References:

(1) Miessler, Fischer & Tarr Inorganic Chemistry

(2) Shriver & Atkins Inorganic Chemistry

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